Method of forming an internal rib in the bore of a tube



1965 w. M. CREIGHTON ETAL 3,213,525

METHOD OF FORMING AN INTERNAL RIB IN THE BORE OF A TUBE Filed Feb. 10, 1961 2 Sheets-Sheet 1 "a if FIG. 4

INVENTQRS W|lham M. Crel hion y SIdIey O. vans Addison KATTORNEY .Mc Murdy Oct. 26, 1965 w. M. CREIGHTON ETAL 3,213,525 METHOD OF FORMING AN INTERNAL RIB IN THE BORE OF A TUBE Filed Feb. 10, 1961 2 Sheets-Sheet 2 FIG. 6

INVENTQRS William M. Crelghron BY Sidley 0. Evans Addison Mc Murdy ATTORNEY United States Patent 3,213,525 METHOD OF FORMING AN INTERNAL RIB IN THE BORE (BF A TUBE William M. Creighton, Sidley 0. Evans, and Addison E.

McMurdy, Beaver Falls, Pa., assignors to The Babcock & Wilcox Company, New York, N.Y., a corporation of New .lersey Filed Feb. 10, 1961, Ser. No. 83,339 4 Claims. (Cl. 29157.3)

This invention relates to a method and apparatus for forming an internal rib in the bore of a tube, and more specifically to a method and apparatus adapted to forming internal helical ribbing in a relatively small bore heat exchanger tube which is capable of containing a pressurized, vaporizable fluid flowing therethrough that is subjected to a high heat flux from a high temperature source.

In the operation of a steam boiler, for example, at subcritical pressures steam is formed in the water inside of the tube, and at successively greater distances along its length there will be an increasing fraction of the flowing fluid in the form of steam and a decreasing fraction in the form of water, depending upon the rate of heat absorption. The change from liquid to vapor occurs both at a solidliquid interface, as at the inside surface of a tube, and at a liquid vapor interface, as with water surrounding steam bubbles. It has been observed that two distinct types of boiling, known as nucleate boiling and film boil ing, can occur at a solid-liquid interface. Nucleate boiling is characterized by the formation and release of steam bubbles on the inside of the heat absorbing surface with the water still wetting the surface, while in film boiling the inside of the heat absorbing surface is covered by a film of steam. To transfer heat from the surface to the fluid a temperature gradient is necessary. The magnitude of this gradient depends mainly on whether nucleate or film boiling is taking place. In nucleate boiling the steam bubbles generated at nucleation points on the heat transfer surface are rapidly detached therefrom and move into the bulk liquid, and the resulting agitation of the mixture produces an excellent heat transfer coefficient. It is well known that the wall metal temperature of a steam generating tube will not rise above the temperature of the contained fluid enough to weaken or otherwise damage the tube so long as the tube is wet with water on the inner wall surface opposite the heat receiving outer surface, i.e. as long as nucleate boiling is taking place, even with high heat transfer rates through the metal of the tube wall due to the contact of hot gases and/or radiation from a furnace. In film boiling a film of steam forms over the heat transfer surface so that steam generation does not occur at the heat transfer surface but at the liquid-vapor interface. The steam film prevents the liquid from wetting the surface and the resulting heat transfer coeflicients are poor. The steam film in film boiling acts as a layer of insulation which retards the heat being transferred from the heat absorbing surface to the water, and therefrom the temperature of the heat absorbing surface rises to a higher level than that resulting from nucleate boiling under the same heat flux and mass flow conditions.

The actual process involved in the formation of the vapor nuclei on the solid-liquid interface surface of a tube is the subject of several interesting but unsubstantiated theories However, observation shows that the nuclei originate at selective points on the surface. As the heat flux across this surface is increased the number of nucleation points increases until the entire surface is covered, thereby replacing the liquid-solid interface with a vapor film. Boiling then proceeds from the liquid-vapor interface. The heat flux at which this vapor film forms is called the break-down point of nucleate boiling, sometimes known as the burn-out point, at which the boiling heat transfer coefficient is not the controlling resistance to heat flux and surface metal oxidization temperatures may be exceeded.

While it has been known that internally ribbed tubes will improve heat transfer, serious fabricating difficulties have been encountered in the manufacture of such tubes. Experience has shown that uniform consistency in the proportions and arrangements of internal lands and grooves defining the ribs in the bore of a heat exchanger tube are vitally important to the satisfactory operation of such tubes. For the critical criteria of such land and groove dimensions, reference is made to a co-pending application SN. 862,232 filed December 28, 1959, now Patent No. 3,088,494. The formation of the ribs, within the dimensional accuracy required, is further aggravated by the fact that such ribs are formed in tube sections of 20 to 30 or more feet in length, with the tubes having a bore size or inside diameter of less than one inch. The forming of internal ribs in such comparatively small bore tubes is further handicapped in that the strength of the holding or tool bar associated with rib forming apparatus is seriously limited by small bore sizes.

Heretofore, a machining operation, such as tapping or threading, was considered to be the most practical method of forming an internal rib in the bore of a tube. However, internally ribbed tubes, manufactured on known tapping or thread making machines, exhibited marked dilferences in the pressure drop in the fluid flowing therethrough, for comparable flow conditions. Also tubes so formed exhibited fluid flow characteristics which were directional, i.e. the measured pressure drop through a given tube for comparable flow conditions was different for flow in one direction through the tubes than for flow in the opposite direction. A study of these pressure drop variations from tube to tube, correlated with the effect of direction of fluid flow through a given tube on pressure drop indicated that the observed pressure drop variations were due to variations in the contour, i.e. the shape and arrangement of the ribs formed throughout the length of the tubes. The asymmetry of internal ribs formed by the conventional tapping or threading operation has been attributed to changes in the rate of feeding of the rib forming tool through the bore of a tube, and in the force applied to the tool such as might be caused by variations of the power supply voltage and in the loading imposed on the motors controlling the relative rotation of the tube with respect to the tool.

Therefore, an object of this invention is to provide both a method and the apparatus whereby a symmetric contour of internal rib formed in a tube can be satisfactorily produced throughout the length of the tube.

Another object is to provide apparatus for forming an internally ribbed tube, the contour of the rib being of such consistency and uniformity that the fluid flow characteristics of the flow channel will be consistent and uniform in either direction of flow.

Another object is to provide a method and apparatus by which a symmetrically shaped rib may be accurately and consistently formed in a bore of tube in an expeditious and relatively simple manner.

Another object is to provide an improved rib forming machine in which the relative rotation of tube with respect to the rib forming tool disposed within the tube bore is synchronized with the feed rate of the tool through its bore so as to minimize irregular or uneven tool travel with respect to the tube.

Still another object of the invention is to provide a method of machining internal ribs in a tube requiring a final bore size which is too small to permit the insertion therein of a tool bar of suflicient strength necessary to resist the torque imparted to the rib forming tool during a rib forming operation.

The foregoing objects and other features and advantages are attained generally by a method comprising the steps of smooth boring the bore of a tube, then forming a helical rib within the smoothed bore by disposing within the bore of the tube a rib forming tool having raised spiral cutting teeth and rotating the tube relative to the tool while resisting the tendency of the tool to rotate therewith, leading the tool axially through the rotating tube, and controlling the feed rate of the tool through the rotating tube so that it is equal to the r.p.m. of the tube times the desired lead of the spiral rib being formed.

Lubrication of the tool or the cutting face is attained by fluid flowing axially through the toolbar and into the connected tool, the latter being provided with ports through which the fluid is discharged within the bore of the tube. Thus the fluid serves both as a lubricant for the rib forming tool and as a flush for evacuating metal removed from the tube wall during the rib forming process.

If desired the height of the rib so formed may be increased by the passage of a modified second rib forming tool through the tube in the same manner as above described, the modified tool removing additional metal from the existing groove defining the internally formed ribs. The modified second tool includes a dummy lead section followed by the cutting section, the dummy section serving to align the tool with the previously formed groove so that the cutting section will, with acceptably close tolerance, remove additional metal from the already formed groove. Controlled sizing, of the tube through a sinking operation, reduces both the inside and outside diameters to the final tube dimensions specified for a specific design condition.

The rib forming step is preferably performed by an apparatus comprising a frame on which there is mounted a tube holder including a spindle for rotatably supporting a tube section in position on the frame, and a carriage means mounted for movement toward and away from one end of the tube. A toolbar, coupled to the carriage for movement therewith, is disposed in axial alignment with the bore of the tube so it can be readily inserted therein. Connected on the end of the toolbar is the rib forming tool having raised spiral cutting teeth. The arrangement is such that the toolbar will lead the tool longitudinally through the bore of the tube during a rib forming operation.

In accordance with this invention a synchronous drive means is provided for positive connecting of the spindle, which rotates the tube, in driving relationship with the lead screw controlling the movement of the carriage, i.e. the feed rate of the tool, through the bore of the tubes. More specifically, the synchronous drive is arranged so as to permit establishing a desired ratio between the spindle r.p.m. and the carriage lead screw such that the carriage will travel in a longitudinal direction at a speed numerically equal to the product of the rpm. of the spindle and the desired lead of the rib. Suitable clutch means are included in the synchronous drive assembly for engaging and disengaging the spindle and the carriage lead screw drive. In addition there is a carriage rapid return means for expediting the rib forming operation on successive tubes by minimizing the time required to return the carriage and toolbar to start position after each rib forming operation.

In accordance with this invention the rib forming tool comprises a shank having a connected cutting head which is fluted to define a plurality of longitudinally extending, circumferentially spaced arcuate surfaces, each of which is provided with transversely extending, raised, spiral forming, cutting teeth. A passageway extending longitudinally of the shank is provided for the flow of a lubricating-flushing fluid therethrough and it communicates with branching ports opening to the fluted portions of the tool through which the lubricant is discharged.

This invention also contemplates a modified tool for increasing the height of ribs formed internally in a tube where a single cutting pass cannot produce the desired rib height specified for a particular design condition. The modified tool is provided with a dummy lead section which precedes the cutting head section. The dummy lead is provided with raised portions arranged to align the tool with the ribs initially formed so that the cutting head, which follows, can be accurately positioned with respect to the existing rib so that the removal of additional metal from the existing groove to increase the rib height can be accurately controlled.

A feature of this invention resides in the provision of a rib forming apparatus having a positive synchronous mechanical drive connection between the rotating spindle and the carriage so as to eliminate variation in the rate of travel of the tool with respect to the speed of the rotating tube.

Another feature of the method and apparatus is the provision whereby the rib forming tool while traveling through the tubes, flushes the metal cuttings out through the finished portion of the tube thereby preventing jamming of the metal chips between the tool and the tube wall.

Another feature resides in the provision of an improved cutting tool which accurately aligns the tool within the existing groove prior to beginning the cut to reincrease the rib height.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which is illustrated and described a preferred embodiment of the invention.

In the drawings:

FIG. 1 illustrates a smooth bore tube, shown partly in section, prior to the formation of the rib therein;

FIG. 2 illustrates the tube, shown partly in section, after a rib forming operation and before a sinking operation;

FIG. 3 illustrates the ribbed tube in accordance with this invention, after the sinking operation;

FIG. 4 is a side elevation of a machine for internally ribbing a tube, in accordance with this invention;

FIG. 5 is an enlarged fragmentary detail illustrating the position of the rib forming tool relative to the tube during a rib forming operation;

FIG. 6 is an enlarged detail view of the rib forming tool in accordance with this invention;

FIG. 7 is an end view of the rib forming end of the tool of FIG. 6;

FIG. 8 is a modified embodiment of a rib forming tool in accordance with this invention.

Referring to the drawings, FIG. 1, illustrates a tube length 20 of circular cross section and formed of suitable carbon or alloy steel of the type particularly suited for use as a heat exchanger tube, for example, a steam generating tube in a steam generator operating at sub-critical pressure. In order to improve the heat transfer characteristics of such a tube, a continuous and symmetrical helical groove 21 is formed in the inner wall of the tube along its length at the same time defining a helical land or rib 22 between the convolutions of grooves 21. While the drawings illustrate a length of tube having a single continuous groove, it is to be understood that the tube may be provided with a plurality of parallel continuous helical grooves. Since the groove and land proportions and dimensions are highly critical, as described in a co-pending application S.N. 862,232 filed December 28, 1959, now Patent No. 3,088,494, it is desirable that tube length 20 first be put through a smooth-boring operation so as to insure a smooth and uniformly finished inner wall surface 20A prior to initiation of the ribbing operaton. An internal rib 22 is then formed by leading a rib forming tool attached to a toolbar through the smooth bore of the tube as the tube is rotated relative to the tool. In his machining operation, depending on the rib height required, the rib may be formed by one or more passes of a rib forming tool through the tube, the tool for each pass being specifically constructed as will be hereinafter described. FIG. 2 illustrates the contour of the rib in a machined tube.

Frequently, design considerations for certain applications require that the final tube bore be less than one inch in diameter. The forming of a rib of uniform contour in a tube having a bore of less than one inch, by a machining operation, is virtually impossible because the small size of the bore limits the size of the toolbar which can be nested n the tube and generally is not able to resist the rotational force of the tool. To obviate this difficulty it is preferred that the rib first be formed in a tube having an initial bore greater than that required in the finished article so that a toolbar of proper proportions, necessary to resist the force tending to rotate the tool may be introduced into the tube. After the formation of the rib in this larger bore tube, as will be herein disclosed, the ribbed tube is then put through a sinking operation to reduce its major inside bore diameter D size to that of the final design proportions (e.g. 0.470 inch), as shown in FIG. 3. The sinking operation when necessary involves the application of compressive forces radially on the outside of the tube which reduces the outside and inside diameter of the tubes, with a corresponding increase in its length. While the sinking operation reduces the outside and inside diameter of the tube, the wall thickness of the tube is not materially effected. For example wall thickness T of the tube of FIGS. 1 and 2 (before sinking) equals substantially the T' of FIG. 3 (after sinking). Likewise, the rib height H of the tube before sinking (FIG. 2) and rib height H of the tube after sinking (FIG. 3) remain substantially equal. However, due to the elongation of the tube occasioned by the sinking operation the width of the grooves 21' and ribs 22 are elongated to account for the added tube length.

Referring to FIG. 4 the machine 30 for producing internal ribs in tube 20 in accordance with this invention includes a frame 31 on which there is supported a tube holder means 32 for maintaining tube 20 in position thereon. The tube holder means 32 includes housing 33 in which a hollow spindle 34 is rotatably supported.

A motor 35, supported on the spindle housing 33, is connected by a belt drive 36 arranged in driving relationship with a drive shaft 37 of a suitable gear train (not shown) which in turn is connected in meshing relationship with the spindle 34 for driving the same. The hollow spindle 34 is adapted to receive one end of the tube and a chuck assembly 33 FIG. 5 which is disposed Within the spindle firmly secures the tube 20 and renders it rotatable with the spindle upon actuation of the motor 35.

Because the tube to be ribbed may be 20, 30 or more feet in length, a tube support 39 is provided at the end of the frame 31 to hold the extending tube in a horizontal position and in axial alignment with the spindle 34. To minimize whipping of tube 20 as it is rotated during a rib forming operation, a plurality of tube rests 40 are longitudinally spaced on the tube spport 39 along the supported length of the tube. Generally, tube rests 40 are in the form of suitable block type bearing members provided with a recess conforming to the contour of the tube resting therein.

Disposed in axial alignment with the spindle housing 33 is a carriage 41 movably mounted on suitable frame ways 42 that form an integral part of the frame 31. Movement of the carriage 41 toward and away from the spindle housing 33 is had by a lead screw 42A which is in threaded engagement with the carriage 41. Coupled to the carriage 41 for movement therewith is a hollow toolbar 43 having a length at least equal to that of the tube to be ribbed. The arrangement is such that the toolbar 43 is coupled at one end to the carriage 41, in axial alignment with the tube 20, while the other, or free, end is positioned within the bore of the tube. Between the spindle housing 33 and the carriage 41 there are disposed a plurality of suitable bearing supports 44 for supporting the toolbar 43. Preferably the toolbar bearing supports 44 are movably mounted on the frame ways 42 so that the spacing between bearing supports along the length of the toolbar can be adjusted to prevent sagging of the bar.

According to this invention a rib forming tool 50 is secured to the free end of the toolbar 43. As shown in FIGS. 6 and 7 the tool consists of a shank 51 having a projecting threaded portion 52 at one end by which it is secured to the free end of the toolbar 43. The shank 51 of the tool is sized a few thousandths of an inch smaller than the bore of the tube to be ribbed. As shown the cutting head portion 53 and connected shank portion 51 of the tool are fluted to define a plurality of longitudinally extending circumferentially spaced land portions 54. Each land portion in the head of the tool is provided with several raised spiral cutting teeth 55 traversing the respective lands. As shown in FIG. 7, the radius of curvature of each raised tooth 55 is such as to permit engagement of the leading edge 55A with the tube interior in order to effect the metal cutting action desired. This action is enhanced by the tooth contour which incorporates a trailing edge 55B.

The leading edge of each land 54 is provided With a longitudinally extending notched offset 56 in which a tool steel bit 57 is secured to form a wear resistant leading edge cutting portion for the respective cutting teeth 55.

Extending longitudinally in the shank of the tool passageway 58 is arranged to communicate with the bore of the hollow toolbar 43 and with branch ports 59 which open to the fluted portion of the shank 51. Fluid flowing through the toolbar 43 and passageway 58, in discharging from ports 59 lubricates the tool during a rib forming operation.

In accordance with this invention a synchronous drive assembly 60 is provided for positively connecting the spindle 34 with the lead screw 42A of the carriage 41 as to effect a driving relationship therebetween and establish a correlation between the rate at which the tool 50 is fed through the tube and the rate of rotation of the tube.

Disposed beween the drive chains 64 and 67 there is disposed a clutch for engaging and disengaging the driving relationship of the spindle 34 and the lead screw 42A.

As shown in FIG. 4, the synchronous drive assembly comprises a power train which includes a sprocket 61 connected to spindle 34 and a sprocket 62 to counter shaft 63. Sprockets 61 and 62 are connected in positive driving relationship through a drive chain 64, with the sprocket 62 on the counter shaft 63 providing the power for the lead screw 42 through a drive chain 65 which engages sprockets 66 and 67 on the counter shaft 63 and lead screw 42A respectively. The gearing of the countershaft drive and the lead screw drive are of selected ratios such that upon engagement of the countershaft clutch 68, the speed ratio established between the spindle 34 and the lead screw 42A will cause the carriage 41 to travel to the right, as viewed in FIG. 4, at a rate equal to the product of the r.p.m. of the spindle 34 and the lead of the rib to be formed. Thus, as the carriage 41 is moving to the right, the toolbar 43 connected thereto will advance the tool through the tube 20, forming the rib as the tube rotates relative to the tool. Because of the positive connection between the spindle 34 and the carriage 41, there will be no variation in the rate of travel of the carriage 41 or tool 50 with respect to the tube 20. For this reason there will be no variations in the contour of the ribs being formed, thus insuring that the desired uniformity and symmetry of the rib will be maintained throughout the entire length of tube.

Lubrication of the tool 50 during a rib forming operation is attained by connecting one end of the hollow toolbar with a lubricating fluid source (not shown) by means of hydraulic line 70, whereby a stream of lubricant is permitted to flow through the toolbar 43 and into the passageways 58 and 59 of the tool 50 from whence the fluid flows into the fluted Ways intermediate the land portions 54, discharging through the space between the tool and the tube, ultimately leaving the tube at the tube end as indicated at 90 in FIG. 5.

To expedite the rib forming operation on successive tubes, 2. rapid carriage return means 71 is mounted on the end 31A of the frame 31 for quick return of the carriage 41 to start position after each rib forming operation. The rapid carriage return means 71 includes a drive motor 72 that is connected by a belt drive 73 with the lead screw 42A of the carriage 41, with a suitable clutch assembly 75 for providing the driving relationship between the motor 72 and the lead screw 42.

With a tube length placed in position on the frame of the machine, the operation of the machine during a rib forming operation is as follows:

The carriage 41 and connected toolbar 43 are advanced to start position so with the toolbar 43 within and extending throughout the length of tube 20, with a portion extending beyond the tube to facilitate attaching the rib forming tool 50 to the end of the toolbar. The drive motor is then energized to rotate the spindle 34 and connected tube 20 relative to the tool, the tube having previously been secured in the tube holder 32. As the countershaft clutch 68 is engaged, the carriage 41 is placed in driving relationship with respect to the spindle 34 through the synchronous drive 60 so that the tool commences to feed through the tube. Thus as the tool advances through the tube 20, the spiral teeth on the tool cutting engage the inner surface of the tube to machine groove 21 therein. As the teeth 55 of the tool begin to cut a spiral groove in the tube, the operation is such that the tool assumes a self feeding feed rate, with the toolbar 43 merely leading the tool through the tube while resisting the torque to prevent tool rotations. Thus an essential feature is that the tool 50 is led or advanced through the tube 20 on the toolbar 43, rather than being pushed or forced therethrough. The significance of this arrangement is that the metal cuttings or chips from the inside surface of the tube can readily be evacuated through the open end of the tube portion already ribbed as seen in FIG. 5. The removal of the chips thus produced is facilitated by the flushing efifect afforded by the lubricant which is being pumped into the tube through the passageways of the hollow toolbar and connected tool and discharged into the space between the tool and tube. The lubricant is pumped into the tube at a pressure suflicient to convey the chips out through the machined back end of the tube, thus preventing the chips from jamming between the tool and the tube. Such jamming, if permitted could result in breaking the tool or the tool bar and in imperfect rib contours.

In the event tube design specifications require a rib height greater than that which can be machined by one pass of the tool, a second pass may be made by a modified tool, which in accordance with this invention will remove more of the metal from the groove which was initially formed causing the first pass of the tool, as herein before described. Because the rib shape and dimensions are highly critical in so far as fluid flow characteristics through the ribbed tube are concerned, it is essential that the tool making the second or other succeeding passes be accurately aligned with the previously machined rib so as to insure that metal from the existing groove only Will be removed.

With reference to FIG. 8, the modified tool contemplated for a second cutting pass is similar in all respects to tool 50 as hereinbefore described, with the exception that a dummy lead section 82 precedes the cutting teeth 81, the latter being sized to remove additional metal from the existing groove; as for example, the machining of an existing groove height of 0.017 inch to say 0.030 inch. The purpose of the dummy lead section 82 is to accurately align the tool 80 with the existing groove so that the cutting teeth 81 which follow, are correctly positioned with respect to the existing groove of the tube. Accordingly, the dummy lead section 82 is provided with raised spiral formed teeth 82A having a dimension slightly less than that of the groove initially formed in the tube and with a pitch equal to that of the groove. Thus, alignment is accurately attained as the teeth 82A of the lead portion 82 are threaded to the initially formed groove of the tube, so that the raised cutting teeth 81 of the modified tool 80, will remove additional metal from the existing groove only.

While the instant invention has been disclosed with reference to a particular embodiment thereof, it is to be appreciated that the invention is not to be taken as limited to or of the details thereof as modifications and variations thereof may be made without departing from the spirit or scope of the invention.

We claim:

1. A method of forming a small diameter ribbed tube having a selected pitch between helical turns of the ribs which comprises the step of forming a helical groove in an oversized tube having an internal diameter greater than the internal diameter of the desired ribbed tube, the step of regulating the formed depth of said helical groove to substantially the same depth as the desired height of the rib in said desired ribbed tube, and the step of drawing said oversized tube to reduce the external and internal diameter of the said tube while substantially maintaining the tube wall thickness, said tube drawing step increasing the pitch of said formed groove in relationship to the reduction of tube diameter.

2. The invention as defined in claim 1 including the step of forming said groove with a cutting tool, and the step of flushing the chips resulting from the said cutting step through the machined end of the tube by a stream of lubricating fluid.

3. A method of internally ribbing a relatively small bore of a heat exchanger tube which is adapted to be subjected to a high heat flux from a high temperature source While conducting therethrough a pressurized vaporizable fluid stream increasing in quality in flowing therethrough comprising the steps of smoothing the bore of a tube, rotating the tube relative to a spiral cutting tool disposed within the bore of the tube, leading the tool axially of the tube as the tube is being rotated, resisting the tendency of the tool to rotate within the bore as the tube is rotated, controlling the axial advance of the tool so that the speed of the tool is equal to the r.p.m. of the tube times the desired lead of the spiral groove being formed by the tool during a rib forming operation, and drawing the ribbed tube to reduce the diameter of the bore and to increase the lead of said spiral groove.

4. A method of internally ribbing a relatively small bore of a heat exchanger tube which is adapted to be subjected to a high heat flux from a high temperature source while conducting therethrough a pressurized vaporizable fluid stream increasing in quality in flowing therethrough comprising the steps of rotating the tube relative to a spiral cutting tool disposed within the bore of the tube, leading the tool axially of the tube as the tube is being rotated to machine a helical groove in the surface of said bore, resisting the tendency of the tool to rotate within the bore as the tube is rotated, controlling the axial advance of the tool so that the speed of the tool is equal to the rpm. of the tube times the desired lead of the spiral rib being formed by the tool during a groove forming operation, and increasing the height of the groove so formed by passing therethrough a second tool constructed to remove additional metal from the existing groove, controlling the axial advance of said second tool in the same manner as that of the first mentioned tool, and drawing the ribbed tube to reduce the diameter of the bore and to increase the lead of said spiral groove.

References Cited by the Examiner UNITED STATES PATENTS 10 10/43 Keema 291.11 12/43 Penner 29157.3 1/45 Lonsdale 80-12 5/48 Gracey 90-28.1 3/49 Rodgers 29157.3 1/57 Edwards 80-12 2/59 Ramsay 29-1.1

FOREIGN PATENTS 9/40 Australia.

WHITMORE A. WILTZ, Primary Examiner.

NEDWIN BERGER, Examiner. 

1. A METHOD OF FORMING A SMALL DIAMETER RIBBED THE HAVING A SELECTED PITCH BETWEEN HELICAL TURNS OF THE RIBS WHICH COMRPRISES THE STEP OF FORMING A HELICAL GROOVE IN AN OVERSIZED TUBE HAVING AN INTERNAL DIAMETER GREATER THAN THE INTERNAL DIAMETER OF THE DESIRED RIBBED TUBE, THE STEP OF REGULATING THE FORMED DEPT OF SAID HELICAL GROOVE TO SUBSTANTIALLY THE SAME DEPTH AS THE DESIRED HEIGHT OF THE RIB IN SAID DESIRED RIBBED TUBE, AND THE STEP OF DRAWING SAID OVERSIZED TUBE TO REDUCE THE EXTERNAL AND INTERNAL DIAMETER OF THE SAID TUBE WHILE SUBSTANTIALLY MAINTAINING THE TUBE WALL THICKNESS, SAID TUBE DRAWING STEP INCREASING THE PITCH OF SAID FORMED GROOVE IN RELATIONSHIP TO REDUCTION OF TUBE DIAMETER. 